1. Technical Field
The present invention relates to an apparatus and a method for detecting wrinkles, twists, tears, etc. of banknotes (bills), sheets of copy paper, plastic sheets, and so on.
2. Description of the Related Art
At the present time, ATMs (Automatic Telling Machines) are widely employed over the world. ATMs often handle fatigued banknotes (having wrinkles, twists, tears, etc.). In such a case, when the banknotes are taken into the ATM and are sorted based on different sorts of the banknotes by a sorting and handling mechanism installed in the ATM, there is a risk that the fatigued banknotes may jam inside the machine. To overcome the jamming problem of fatigued banknotes, highly fatigued banknotes are prevented from going back to the market. For example, banknote counters and ATMs at banks can be provided with a function of detecting and indentifying highly fatigued banknotes.
An example of a known system for detecting fatigue of a banknote etc. is described in Japanese Unexamined Patent Application Publication No. 5-97284 (“the '284 application”). In this detection system, an ultrasonic sending sensor and an ultrasonic receiving sensor are provided on the principal surface side of a medium to measure ultrasonic waves reflected by the medium. A surface condition (e.g., wrinkles) of the medium is detected by comparing the medium with previously input information about the distance between the medium and the sensor.
However, detection system described in the '869 application has problems because information about the distance between the medium and the sensor must be input in advance, and processing operations are complicated. Further, if the medium's position deviates from its expected position, for example, it becomes difficult to accurately measure the distance between the medium and the sensor.
In another example, Japanese Unexamined Patent Application Publication No. 2006-250869 (“the '869 application”) describes a detection apparatus in which a banknote is guided to pass between an ultrasonic sending unit and an ultrasonic receiving unit. The banknote resonates upon irradiation of ultrasonic waves by the ultrasonic sending unit, and secondary radiation waves are received to determine a natural frequency of the banknote from the resonance frequency. The determined natural frequency is compared with the natural frequency of a normal banknote to recognize deterioration in rigidity of the banknote.
However, the detection apparatus described in the '869 application also has problems. The natural frequency of a normal banknote must be input in advance. In order to inspect plural kinds of banknotes by a single apparatus, therefore, plural types of data must be input to execute comparative processing operations for each type. Thus, versatility of the apparatus described in the '869 application is insufficient.